package utils

import "math/bits"

/*
Copyright (c) 2009 The Go Authors. All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

  * Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
  * Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following disclaimer
in the documentation and/or other materials provided with the
distribution.
  * Neither the name of Google Inc. nor the names of its
contributors may be used to endorse or promote products derived from
this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

*/

const fastSmalls = true // enable fast path for small integers

// AppendInt appends the string form of the integer i,
// as generated by FormatInt, to dst and returns the copied size.
func AppendInt(dst []byte, i int64, base int) int {
	if fastSmalls && 0 <= i && i < nSmalls && base == 10 {
		return small(dst, int(i))
	}
	return formatBits(dst, uint64(i), base, i < 0)
}

// small returns the string for an i with 0 <= i < nSmalls.
func small(dst []byte, i int) int {
	if i < 10 {
		return copy(dst, digits[i:i+1])
	}

	return copy(dst, smallsString[i*2:i*2+2])
}

const nSmalls = 100

const smallsString = "00010203040506070809" +
	"10111213141516171819" +
	"20212223242526272829" +
	"30313233343536373839" +
	"40414243444546474849" +
	"50515253545556575859" +
	"60616263646566676869" +
	"70717273747576777879" +
	"80818283848586878889" +
	"90919293949596979899"

const host32bit = ^uint(0)>>32 == 0

const digits = "0123456789abcdefghijklmnopqrstuvwxyz"

// formatBits computes the string representation of u in the given base.
// If neg is set, u is treated as negative int64 value. If append_ is
// set, the string is appended to dst and the resulting byte slice is
// returned as the first result value; otherwise the string is returned
// as the second result value.
//
func formatBits(dst []byte, u uint64, base int, neg bool) int {
	if base < 2 || base > len(digits) {
		panic("strconv: illegal AppendInt/FormatInt base")
	}
	// 2 <= base && base <= len(digits)

	i := len(dst)

	if neg {
		u = -u
	}

	// convert bits
	// We use uint values where we can because those will
	// fit into a single register even on a 32bit machine.
	if base == 10 {
		// common case: use constants for / because
		// the compiler can optimize it into a multiply+shift

		if host32bit {
			// convert the lower digits using 32bit operations
			for u >= 1e9 {
				// Avoid using r = a%b in addition to q = a/b
				// since 64bit division and modulo operations
				// are calculated by runtime functions on 32bit machines.
				q := u / 1e9
				us := uint(u - q*1e9) // u % 1e9 fits into a uint
				for j := 4; j > 0; j-- {
					is := us % 100 * 2
					us /= 100
					i -= 2
					dst[i+1] = smallsString[is+1]
					dst[i+0] = smallsString[is+0]
				}

				// us < 10, since it contains the last digit
				// from the initial 9-digit us.
				i--
				dst[i] = smallsString[us*2+1]

				u = q
			}
			// u < 1e9
		}

		// u guaranteed to fit into a uint
		us := uint(u)
		for us >= 100 {
			is := us % 100 * 2
			us /= 100
			i -= 2
			dst[i+1] = smallsString[is+1]
			dst[i+0] = smallsString[is+0]
		}

		// us < 100
		is := us * 2
		i--
		dst[i] = smallsString[is+1]
		if us >= 10 {
			i--
			dst[i] = smallsString[is]
		}

	} else if isPowerOfTwo(base) {
		// Use shifts and masks instead of / and %.
		// Base is a power of 2 and 2 <= base <= len(digits) where len(digits) is 36.
		// The largest power of 2 below or equal to 36 is 32, which is 1 << 5;
		// i.e., the largest possible shift count is 5. By &-ind that value with
		// the constant 7 we tell the compiler that the shift count is always
		// less than 8 which is smaller than any register width. This allows
		// the compiler to generate better code for the shift operation.
		shift := uint(bits.TrailingZeros(uint(base))) & 7
		b := uint64(base)
		m := uint(base) - 1 // == 1<<shift - 1
		for u >= b {
			i--
			dst[i] = digits[uint(u)&m]
			u >>= shift
		}
		// u < base
		i--
		dst[i] = digits[uint(u)]
	} else {
		// general case
		b := uint64(base)
		for u >= b {
			i--
			// Avoid using r = a%b in addition to q = a/b
			// since 64bit division and modulo operations
			// are calculated by runtime functions on 32bit machines.
			q := u / b
			dst[i] = digits[uint(u-q*b)]
			u = q
		}
		// u < base
		i--
		dst[i] = digits[uint(u)]
	}

	// add sign, if any
	if neg {
		i--
		dst[i] = '-'
	}

	return copy(dst, dst[i:])
}

func isPowerOfTwo(x int) bool {
	return x&(x-1) == 0
}
